1. Field of the Invention
This invention relates to a picture image scanning and reproducing method suitable for practice in color scanners for graphic arts or facsimiles and a system therefor.
2. Description of the Prior Art
In a color scanner, an original is scanned to obtain picture image signals by photoelectric conversion means, the picture image signals are sampled into quantized signals, and the thus-quantized signals are then subjected to a prescribed digital picture image processing.
In each of conventional color scanners, it was necessary to determine the sampling speed (i.e., the sampling frequency) by the reproduced magnification only because among various conditions for picture image reproduction, the sampling unit for each picture pattern, in other words, the shape of each picture element was not required to be a square. It was thus unnecessary to change the sampling speed even when the line width was varied.
It is however extremely desirable that the shape of each picture element is a square if one wants, like in a layout scanner, to store picture elements, which have been sampled out and then subjected to a color correction or the like, temporarily in a large-capacity memory such as disk memory or the like and thereafter to carry out rotation or the like of the picture image at a layout processing unit.
This will hereinafter be described in further detail. FIG. 1 is a drawing showing some sampling units. In FIG. 1, the arrow indicates the main-scanning direction in which the scanning takes place upon rotation of a drum. FIGS. 1(a) and 1(b) illustrate sampling units pertaining to conventional color scanners. Supposing now that scanning lines are drawn at a density of L' lines/cm, the line width then has a dimension determined by 10/L'=w (mm). This width is changed in accordance with the level of resolution required for the reproduction of each picture image and the screen ruling. Let's now suppose that a picture image is scanned by two types of scanning lines, one having a width w.sub.1 and the other a width w.sub.2. Conventionally, it was not required to change the sampling pitch e in the scanning direction. Thus, it was allowed to keep the sampling pitch always constant. This is however not the case when square-shaped picture elements are required as mentioned above. As depicted in FIGS. 1(c) and 1(d), it is necessary to change the sampling pitch to w.sub.1 or w.sub.2 in the scanning direction as the line width changes to w.sub.1 or w.sub.2 which is the same as the sampling pitch.
The above change is required in order to follow changes in the screen ruling and the number of scanning lines. Here, there has been developed another problem that the picture image processing unit has to be operated at a different throughput in response to the above-mentioned change to the sampling pitch.
In other words, it becomes necessary to feed different timing signals to respective operation modules in accordance with given requirements as the transferring speed of picture image data to be processed changes.
It is however difficult from the practical viewpoint to generate a variety of timing signals in accordance with various conditions for separation at a timing clock generation circuit and then to send them to the respective operation modules, because many timing signal lines are required and clock signals are thus all delayed.
No particular problem has conventionally been developed where rather fewer operation modules were employed, for example, in such a color scanner that calculations for all color corrections were conducted in accordance with a three-dimensional lookup table using B-, G- and R-signals as inputs. However, the above method becomes more difficult to practice as more operation modules making use of multi-CPU equipped with various functions or like devices are employed. Let's now suppose by way of example that three types of clock signals are required for each operation module. Fifteen types of clock signals are required where there are five modules. If it is necessary to switch over 15 types of basic timings from one to another in accordance with separation conditions, the clock generation circuit is required to produce 15 types of clock signals, each in 15 different ways. This renders the circuit enormous.
It may be contemplated to incorporate a timing unit in each operation module per se, which timing unit is adapted to produce a desired clock from a system clock. Each of such operation modules produces such a desired clock by either dividing or multiplying the system clock. Therefore, many types of system clocks are required. In the case of multiplication in particular, multiplication circuits are required as many as the number of operation modules relying upon multiplication of the system clock. It may be possible to produce such many types of system clocks by the clock generation circuit and then to feed them respectively to the operation modules. However, this renders the clock generation circuit too complex for its actual application.
Therefore, it is extremely desirable to design the picture image processing unit to operate at a constant throughput in every situation.